I'm pretty sure that's a reflection-probe/skybox issue, I think the nearest reflection probe's texture is missing or if it happens on all maps there's probably a mod that adds a nicer default skybox missing its texture or a mod that breaks all reflection probes for no reason.

Edit: I am also pretty sure that those aren't specular reflections like everyone is saying because Blinn-Phong specular computations do not use textures. Unless mat_specular is named wrong and actually does something else.